Method for regenerating a zeolite catalyst

ABSTRACT

A process for the regeneration of a zeolite catalyst which comprises treating the catalyst thermally in the presence of a gas stream at temperatures above 120° C., the weight-based residence time of the gas stream over the catalyst during the thermal treatment being greater than 2 hours.

CONTINUING APPLICATION DATA

This application is a 371 of international application No.PCT/EP01/10490, filed on Sep. 11, 2001.

The present invention relates to a process for the regeneration of azeolite catalyst and to an integrated process for the preparation of anepoxide as part of which the regeneration according to the invention iscarried out.

It is known from the prior art that the catalytic activity ofheterogeneous catalysts for the oxidation of organic compounds in theliquid phase, where, in particular, the epoxidation of organic compoundshaving at least one C—C double bond using a hydroperoxide in thepresence of a zeolite catalyst is of importance, decreases withadvancing experiment time, and the corresponding catalysts then have tobe regenerated.

Accordingly, processes for the regeneration of zeolite catalysts arealready known from the prior art. In this respect, we refer to WO98/55228 and the prior art cited therein. Within this prior art,basically two different procedures for catalyst regeneration areproposed.

-   -   1. If the catalyst is employed in suspension, it is firstly        separated from the liquid reaction product and transferred into        a regeneration device suitable for regeneration, where it is        regenerated by thermal treatment in the presence of oxygen;    -   2. If the catalyst is employed as a fixed bed, the liquid phase        is discharged or pumped off, and the catalyst is regenerated        either in the reactor itself or in another regeneration device        by thermal treatment in the presence of oxygen.

WO 98/18556 discloses a process for the regeneration of a titaniumsilicalite catalyst in which the catalyst is flushed with a gas streamat a temperature of at least 130° C. in such a way that the weight-basedresidence time of the gas stream over the catalyst is less than 2 hours.

In addition, regeneration by treatment of the catalyst with a liquidwhich is in turn an oxidant, for example hydrogen peroxide, at elevatedtemperature has already been described a number of times in the priorart. In this respect, we refer to DE-A 195 28 220 and WO 98/18555.

In view of this prior art, it is an object of the present invention toprovide a further improved, in particular more effective process for theregeneration of zeolite catalysts which can readily be integrated intocontinuous and integrated processes for the preparation of epoxides ofthe type in question here, and which results, in particular, in theopening or re-closure of the reactors without long shutdown and downtimes. In particular, this process should be suitable for theregeneration of zeolite catalysts which are employed in an oxidation inthe fixed-bed process.

In particular, it should be taken into account here that during theregeneration of a fixed bed, the pressure loss in the reactor is a veryimportant parameter. Excessively high pressure losses can result inmechanical damage to the catalyst.

We have found that this and further objects are achieved by the processaccording to the invention for the regeneration of a zeolite catalyst.

The present invention accordingly relates to a process for theregeneration of a zeolite catalyst which comprises treating the catalystthermally in the presence of a gas stream at temperatures above 120° C.,the weight-based residence time of the gas stream over the catalystduring the thermal treatment being greater than 2 hours.

The following is a brief description of the figures of the presentapplication:

FIG. 1: results obtained in Example 1 of the present application;

FIG. 2: results obtained in Example 2 of the present application; and

FIG. 3: results obtained in Example 3 of the present application.

The term “weight-based residence time” used in accordance with theinvention denotes the ratio of the catalyst weight (M_(cat)) divided bythe mass flow rate (M_(gases)) of the gases used in the regeneration.

The regeneration according to the invention is carried out in such a waythat the weight-based residence time of the regeneration gas is greaterthan 2 hours, preferably from 3 to 10 hours and particular preferablyfrom 4 to 6 hours.

The process is generally carried out in such a way that the pressureloss over the reactor is not greater than 4 bar, preferably not greaterthan 3 bar and in particular not greater than 2.5 bar.

In the process according to the invention, it is possible to regenerateboth catalysts in powder form, which are used as a suspension, and alsocatalysts packed in a fixed bed in the form of moldings, for example aspellets or extrudates, and on meshes, for example stainless steel,kanthal, or packings of crystallized catalysts and coated catalystsconsisting of an inert core of SiO₂, α-Al₂O₃, highly calcined TiO₂ orsteatite and an active catalyst shell comprising a zeolite.

If the catalyst has been used in the suspension process, it must firstbe separated from the reaction solution by a separation step, forexample filtration or centrifugation. The at least partiallydeactivated, pulverulent catalyst obtained in this way can then be fedto regeneration. The steps carried out at elevated temperature duringthe regeneration process are preferably carried out in revolving tubularfurnaces in the case of pulverulent catalysts of this type. In theregeneration of a catalyst used in the suspension process, it isparticularly preferred for some of the at least partially deactivatedcatalyst to be removed continuously as part of coupling of the reactionin the suspension process and the regeneration process according to theinvention, and regenerated externally by means of the process accordingto the invention, and for the regenerated catalyst to be fed back intothe reaction in the suspension process.

Besides the regeneration of catalysts in powder form, the processaccording to the invention can also be used for the regeneration ofcatalysts as moldings, for example those packed in a fixed bed. In theregeneration of a catalyst packed in a fixed bed, the regeneration ispreferably carried out in the reaction apparatus itself; to do this,there is no need to remove or install the catalyst, and consequentlythere is no additional mechanical loading at all. In the regeneration ofthe catalyst in the reaction apparatus itself, the reaction is firstlyinterrupted, any reaction mixture present is removed, the regenerationis carried out, and the reaction is then continued.

The regeneration according to the invention proceeds in an essentiallyidentical manner both in the regeneration of pulverulent catalysts andin the regeneration of catalysts in shaped form.

However, the regeneration process according to the invention isparticularly suitable for regeneration in a fixed-bed reactor, inparticular in a tubular reactor or tube-bundle reactor. The terms“tubular reactor” and “tube-bundle reactor” here describe combinedparallel arrangements of a multiplicity of channels in the form oftubes, where the tubes can have any desired cross section. The tubes arearranged in a fixed spatial relationship to one another, are preferablyspatially separated from one another and are preferably surrounded by ajacket which covers all tubes. This enables, for example, a heating orcooling medium to be passed through the jacket, so that the temperatureof all tubes is controlled uniformly.

The individual tubes within the tubular or tube-bundle reactorpreferably used furthermore preferably have a length of fromapproximately 0.5 to 15 m, further preferably from 5 to 15 m and inparticular from approximately 8 to 12 m.

The catalyst should preferably remain in the reactor during theregeneration. Furthermore, the regeneration process according to theinvention can also be used for zeolite catalysts used in a plurality ofreactors connected in parallel or in series or (in some cases) inparallel and in series.

The regeneration according to the invention is carried out attemperatures above 120° C., preferably above 350° C. and in particularat from 400° C. to 650° C.

There are in principle no restrictions regarding the regeneration gasesused so long as the regeneration can be carried out in such a way thatthe catalyst in the interior of the reactor does not heat up, forexample due to burn-off of the organic coatings thereon, in such a waythat the pore structure thereof and/or the reactor itself is damaged.The regeneration is preferably carried out in such a way that a hot-spotwhich forms a temperature increase of from 10 to 30° C., preferably notmore than 20° C., forms within the catalyst bed.

Accordingly, suitable regeneration gases are oxygen-containingregeneration gases, for example air, and gases which are essentiallyfree from oxygen, oxygen-supplying compounds and other oxidizingconstituents. If the regeneration gas comprises oxygen, its proportionin the regeneration gas is preferably less than 20% by volume, furtherpreferably from 0.1 to 10% by volume, in particular from 0.1 to 5% byvolume and still further preferably from 0.1 to 2% by volume of oxygen.Preference is given to a mixture of air and corresponding volumes ofnitrogen.

The term “oxygen-supplying substances” used above covers all substanceswhich are capable of releasing oxygen or removing carbon-containingresidues under the stated regeneration conditions. Particular mentionshould be made of the following:

Nitrogen oxides of the formula N_(x)O_(y), where x and y are selected soas to give a neutral nitrogen oxide, N₂O, N₂O-containing offgas streamfrom an adipic acid plant, NO, NO₂, ozone, CO, CO₂ or a mixture of twoor more thereof. On use of carbon dioxide as oxygen-supplying substance,the regeneration is carried out at a temperature in the range from 500°C. to 800° C.

There are no particular restrictions regarding the zeolite catalystsregenerated in the course of the present process.

As is known, zeolites are crystalline aluminosilicates having orderedchannel and cage structures which have micropores which are preferablyless than approximately 0.9 nm. The network of such zeolites is built upfrom SiO₄ and AlO₄ tetrahedra, which are linked via common oxygenbridges. An overview of the known structures is given, for example, inW. M. Meier, D. H. Olson and Ch. Baerlocher, “Atlas of Zeolite StructureTypes”, Elsevier, 4^(th) Edn., London, 1996.

Zeolites are also known which contain no aluminum and in which some ofthe Si(IV) in the silicate lattice has been replaced by titanium in theform of Ti(IV). These titanium zeolites, in particular those having acrystal structure of the MFI type, and methods for their preparation aredescribed, for example, in EP-A 0 311 983 and EP-A 405 978. In additionto silicon and titanium, such materials may also contain additionalelements, for example aluminum, zirconium, tin, iron, cobalt, nickel,gallium, boron or small amounts of fluorine. In the zeolite catalystspreferably regenerated by means of the process according to theinvention, some or all of the titanium of the zeolite may have beenreplaced by vanadium, zirconium, chromium or niobium or a mixture of twoor more thereof. The molar ratio between titanium and/or vanadium,zirconium, chromium or niobium to the total of silicon and titaniumand/or vanadium and/or zirconium and/or chromium and/or niobium isgenerally in the range from 0.01:1 to 0.1:1.

Titanium zeolites, in particular those having a crystal structure of theMFI type, and methods for their preparation are described, for example,in WO 98/55228, WO 98/03394, WO 98/03395, EP-A 0 311 983 and EP-A 0 405978, which are expressly incorporated into the present invention by wayof reference in their full scope in this respect.

Titanium zeolites having an MFI structure are known for the fact thatthey can be identified via a certain pattern in the determination oftheir X-ray diffraction diagrams and in addition via a skeletalvibration band in the infrared region (IR) at about 960 cm⁻¹ and thusdiffer from alkali metal titanates or crystalline or amorphous TiO₂phases.

Suitable here are, in detail, titanium-, germanium-, tellurium-,vanadium-, chromium-, niobium- and zirconium-containing zeolites havinga pentasil zeolite structure, in particular the types with X-rayassignment to the ABW, ACO, AEI, AEL, AEN, AET, AFG, AFI, AFN, AFO, AFR,AFS, AFT, AFX, AFY, AHT, ANA, APC, APD, AST, ATN, ATO, ATS, ATT, ATV,AWO, AWW, BEA, BIK, BOG, BPH, BRE, CAN, CAS, CFI, CGF, CGS, CHA, CHI,CLO, CON, CZP, DAC, DDR, DFO, DFT, DOH, DON, EAB, EDI, EMT, EPI, ERI,ESV, EUO, FAU, FER, GIS, GME, GOO, HEU, IFR, ISV, ITE, JBW, KFI, LAU,LEV, LIO, LOS, LOV, LTA, LTL, LTN, MAZ, MEI, MEL, MEP, MER, MFI, MFS,MON, MOR, MSO, MTF, MTN, MTT, MTW, MWW, NAT, NES, NON, OFF, OSI, PAR,PAU, PHI, RHO, RON, RSN, RTE, RTH, RUT, SAO, SAT, SBE, SBS, SBT, SFF,SGT, SOD, STF, STI, STT, TER, THO, TON, TSC, VET, VFI, VNI, VSV, WEI,WEN, YUG or ZON structure and to mixed structures consisting of two ormore of the above-mentioned structures. Also feasible for use in theprocess according to the invention are titanium-containing zeoliteshaving the ITQ-4, SSZ-24, TTM-1, UTD-1, CIT-1 or CIT-5 structure.Further titanium-containing zeolites which may be mentioned are thosehaving the ZSM-48 or ZSM-12 structure.

Ti zeolites having the MFI, MEL or MFI/MEL mixed structure are regardedas particularly preferred for the process according to the invention.Preference is furthermore given, in detail, to the Ti-containing zeolitecatalysts generally known as “TS-1”, “TS-2” and “TS-3”, and Ti zeoliteshaving a skeletal structure which is isomorphous with β-zeolites.

Accordingly, the present invention also relates to a process asdescribed above wherein the catalyst is a titanium silicalite of thestructure TS-1.

The term “alkene” as used for the purposes of the present invention istaken to mean all compounds which have at least one C—C double bond.

The following alkenes may be mentioned as examples of such organiccompounds having at least one C—C double bond:

Ethene, propene, 1-butene, 2-butene, isobutene, butadiene, pentenes,piperylene, hexenes, hexadienes, heptenes, octenes, diisobutene,trimethylpentene, nonenes, dodecene, tridecene, tetra- to eicosenes,tri- and tetrapropene, polybutadienes, polyisobutenes, isoprenes,terpenes, geraniol, linalool, linalyl acetate, methylenecyclopropane,cyclopentene, cyclohexene, norbornene, cycloheptene, vinylcyclohexane,vinyloxirane, vinylcyclohexene, styrene, cyclooctene, cyclooctadiene,vinylnorbornene, indene, tetrahydroindene, methylstyrene,dicyclopentadiene, divinylbenzene, cyclododecene, cyclododecatriene,stilbene, diphenylbutadiene, vitamin A, beta-carotene, vinylidenefluoride, allyl halides, crotyl chloride, methallyl chloride,dichlorobutene, allyl alcohol, methallyl alcohol, butenols, butenediols,cyclopentenediols, pentenols, octadienols, tridecenols, unsaturatedsteroids, ethoxyethene, isoeugenol, anethol, unsaturated carboxylicacids, for example acrylic acid, methacrylic acid, crotonic acid, maleicacid and vinylacetic acid, unsaturated fatty acids, for example oleicacid, linoleic acid and palmitic acid, and naturally occurring fats andoils.

In the process according to the invention, preference is given toalkenes which contain 2 to 8 carbon atoms. Particular preference isgiven to ethene, propene and butene. Especial preference is given topropene.

Accordingly, the present invention also relates to a process asdescribed above or to an integrated process as described above whereinthe alkene is propene.

The term “hydroperoxide” covers all hydroperoxides including hydrogenperoxide, reference being made to the prior art with respect to thehydroperoxide solutions which can be used for the purposes of theprocess according to the invention and their preparation. To this end,we refer by way of example to DE 197 23 950.1 and the prior art citedtherein.

For the preparation of the hydrogen peroxide used, recourse can be made,for example, to the anthraquinone process, by which virtually all thehydrogen peroxide produced worldwide is prepared. This process is basedon the catalytic hydrogenation of an anthraquinone compound to give thecorresponding anthrahydroquinone compound, subsequent reaction thereofwith oxygen to form hydrogen peroxide, and subsequent removal of theresultant hydrogen peroxide by extraction. The catalysis cycle is closedby re-hydrogenation of the reformed anthraquinone compound.

An overview of the anthraquinone process is given in “Ullmann'sEncyclopedia of Industrial Chemistry”, 5^(th) Edition, Volume 13, pages447 to 456.

It is likewise conceivable to obtain hydrogen peroxide by convertingsulfuric acid into peroxodisulfuric acid by anodic oxidation withsimultaneous cathodic evolution of hydrogen. The hydrolysis of theperoxodisulfuric acid via peroxosulfuric acid then gives hydrogenperoxide and sulfuric acid, which is thus recovered. It is of coursealso possible to prepare hydrogen peroxide from the elements.

Before use of hydrogen peroxide in the process according to theinvention, it is possible, for example, to free a commercially availablehydrogen peroxide solution from undesired ions. Conceivable methods hereare, inter alia, those as described, for example, in WO 98/54086, DE-A42 22 109 and WO 92/06918. It is likewise possible for at least one saltpresent in the hydrogen peroxide solution to be removed from thehydrogen peroxide solution by ion exchange by means of an apparatuswhich contains at least one non-acidic ion exchanger bed having a flowcross-sectional area A and a depth D, where the depth D of the ionexchanger bed is less than or equal to 2.5·A^(1/2) and in particularless than or equal to 1.5·A^(1/2). For the purposes of the presentinvention, it is in principle possible to employ any non-acidic ionexchanger beds containing cation exchangers and/or anion exchangers.Cation and anion exchangers can also be used as so-called mixed bedswithin a single ion exchanger bed. In a preferred embodiment of thepresent invention, only one type of non-acidic ion exchanger isemployed. The use of basic ion exchange, particularly preferably that ofa basic ion exchanger and further particularly preferably that of aweakly basic anion exchanger, is furthermore preferred.

In a particularly preferred embodiment, the present invention relates toa process for the regeneration of a zeolite catalyst which comprises thefollowing steps (1) to (4):

-   -   (1) washing the zeolite catalyst with a solvent    -   (2) drying the washed zeolite catalyst at a temperature of from        −50 to 250° C.    -   (3) heating the dried catalyst    -   (4) regeneration the heated catalyst by means of a process        according to the present invention.

This preferred regeneration process furthermore preferably comprises thefurther steps (5) and/or (6):

-   -   (5) cooling the regenerated catalyst obtained in step (4)    -   (6) conditioning the catalyst obtained in step (4) or in step        (5).

These steps are now described again individually in detail. It shouldfirst be noted that the zeolite catalyst to be regenerated is generallya catalyst which is employed in the oxidation of an alkene by reactionof the alkene with a hydroperoxide, preferably a reaction which has beencarried out continuously, and as a consequence of a drop in activity nowhas to be regenerated. As already indicated above, the regenerationaccording to the invention is preferably carried out in the reactor(s)in which the reaction of the alkene with a hydroperoxide in the presenceof the catalyst to be regenerated is also carried out.

In a further, very particularly preferred embodiment, the reactor isoperated as an integrated system with the work-up of the target productand the regeneration according to the invention, since this procedureallows a closed loop of solvent.

-   (1) Washing the Zeolite Catalyst With a Solvent    -   The first step in this embodiment of the regeneration according        to the invention firstly comprises washing the deactivated        catalyst with a solvent. For this purpose, firstly the supply of        the starting materials of hydroperoxide and organic compound is        interrupted. Solvents which can be employed here are all        solvents in which the respective reaction product of the        oxidation of the alkene is readily soluble. Preference is given        to solvents of this type selected from the group consisting of        water, an alcohol, preferably methanol, an aldehyde, an acid,        for example formic acid, acetic acid or propionic acid, a        nitrile, a hydrocarbon and a halogenated hydrocarbon. For        details on solvents of this type, reference is made to WO        98/55228, which is expressly incorporated into the present        invention by way of reference in its full scope in this respect.    -   Preference is given to solvents which are already employed in        the reaction, i.e., for example, function as solvent in the        epoxidation of olefin using the catalyst to be regenerated.        Mention may be made by way of example as such for the        epoxidation of olefins of the following: water, alcohols, for        example methanol, ethanol, 1-propanol, 2-propanol,        2-methyl-2-propanol, 1-butanol, 2-butanol, allyl alcohol or        ethylene glycol, or ketones, for example acetone, 2-butanone,        2-methyl-3-butanone, 2-pentanone, 3-pentanone,        2-methyl-4-pentanone or cyclohexanone.    -   If the solvent employed for the washing is the solvent already        used in the reaction, its feed is continued, and the catalyst is        washed with the solvent at a temperature of, in general, from 40        to 200° C., if desired with increasing temperature and under        pressure. The washing is preferably continued until the content        of reaction product in the discharge drops to less than 1% of        the initial value. If another solvent is to be used, the feed of        hydroperoxide, the reaction product and the solvent in the        reaction is interrupted, and the feed of the solvent for washing        is started. Particular preference is given to the use of the        same solvent for the reaction and for the washing of the        catalyst.    -   There are no restrictions at all regarding the duration of the        washing process, relatively long washing times and thus very        substantial removal of the reaction product or the organic        coatings being advantageous.-   (2) Drying of the Washed Zeolite Catalyst at a Temperature of from    −50 to 250° C.    -   After the washing of the catalyst, the solvent used is        discharged or pumped out of the reactor. The porous catalyst        then still contains considerable amounts of adhering solvent,        which is substantially removed by drying with a gas stream at        temperatures of from −50 to 250° C., the temperature used being        in the vicinity of the boiling point of the solvent at the        pressure desired in each case. The temperatures are typically in        the region of ±50° C. above or below the boiling point.    -   The drying is generally carried out using an inert gas, for        example nitrogen, argon, CO₂, hydrogen, synthesis gas, methane,        ethane or natural gas. Preference is given to nitrogen. The        solvent-charged gas is then either disposed of, for example by        incineration using a flare, or fed in at a suitable point, for        example during work-up of the reaction product from the process        for the oxidation of an alkane, and the solvent present therein        is recovered.    -   In a preferred embodiment, the washing is carried out under        pressure at a temperature above the boiling point of the solvent        and, after the solvent has been discharged, the pressure is        reduced to such an extent that some of the solvent already        evaporates due to the latent heat of the reactor even before or        during the supply of gas for the drying.    -   For the supply of heat within this step, either a gas or a        liquid can be employed, for example within the jacket of a        tubular reactor. Preference is given to the use of a liquid for        the temperature range below 150° C. and to a gas for the        temperature range above 150° C.-   (3) Heating of the Dried Catalyst    -   After the drying, the catalyst to be regenerated is heated. This        heating can be carried out by any methods familiar to the person        skilled in the art, the heating preferably being carried out in        the presence of a stream of inert gas, for example nitrogen,        argon, methane, ethane or natural gas.    -   In a preferred embodiment of the process according to the        invention, the catalyst is located in the tubes of a tube-bundle        reactor. In reactors of this type, the heat is introduced into        the system through the jacket space. The heating rate here must        be selected so that inadmissibly high mechanical stresses do not        arise in the reactor. Typical heating rates are from 0.01°        C./min to 0.2° C./min.-   (4) Regeneration of the Heated Catalyst by Means of a Process    According to the Present Invention    -   The regeneration of the catalyst is subsequently carried out as        described in detail in the present application.-   (5) Cooling of the Regenerated Catalyst Obtained in Step (4)    -   After completion of the regeneration in step (4), the        regenerated catalyst, preferably the entire reactor with the        regenerated catalyst located therein, can be cooled to a        temperature of preferably below 200° C.-   (6) Conditioning of the Catalyst Obtained in Step (4) or Step (5)    -   After the regeneration according to the invention or the        cooling, the catalyst can also be conditioned in order to        dissipate the heat of sorption of the solvent or starting        materials in a controlled manner before re-use of the catalyst.        To this end, small amounts of a solvent, preferably the same        solvent which has been employed for the reaction or for the        washing of the catalyst, in particular an alcohol, for example        methanol, are admixed with the inert gas flowing past the        catalyst, and the solvent vapor-containing stream of inert gas        is passed through the catalyst bed. The solvent content and the        volume flow rate of the conditioning gas are selected so that an        inadmissible peak temperature of the catalyst does not occur.        The temperature increase should preferably be not greater than        100° C. above the mean temperature of the heat transfer medium,        for example in a jacket space of a tubular reactor.    -   After the heat liberation has subsided, the feed of conditioning        gas containing solvent is interrupted, and the reactor,        preferably the fixed-bed reactor, is filled with liquid and put        back into operation.

In the optional steps (5) and (6) of the process according to theinvention, it is important that both the cooling is not carried out tooquickly and that the conditioning is not carried out too quickly, sinceboth processes can have adverse effects on the catalyst bed in thereactor. In addition, an excessively fast temperature increase withinthe catalyst during conditioning should also be avoided for the samereasons.

The regenerated catalyst is preferably, as indicated above, re-employedfor the reaction of the alkene with the hydroperoxide. In particular,the regeneration according to the invention or the integrated processfor the oxidation of an alkene can be used for the conversion ofpropylene into propylene oxide by means of hydrogen peroxide, furtherpreferably in methanol solution.

The process according to the invention has, in particular, the followingadvantages:

-   -   the gentle way in which the reaction is carried out means that        zeolite catalysts can be regenerated in such a way that the        activity after regeneration is substantially retained;    -   the regeneration process according to the invention can, on use        of a fixed-bed catalyst, be carried out in the reactor itself        without removal of the catalyst;    -   the solvents employed in the regeneration process according to        the invention may be identical with the solvents during the        reaction and overall be circulated completely.

The invention will now be explained in greater detail with reference tosome examples according to the invention.

EXAMPLES Example 1

A TS-1 catalyst (in the form of 1.5 mm pellets) was introduced to a beddepth of 8 m (in total 4480 g of catalyst) into a tube with a length of1.25 m open at the top and with electrical secondary heating. By meansof a calibrated mass flow meter, various mass flow rates of nitrogenwere passed through the reactor at room temperature and at 400° C., andthe corresponding pressure loss over the bed depth measured. The resultsare shown in FIG. 1 as pressure loss vs. mass-based residence time. Itcan be seen that for mass-based residence times of less than 2 hours,the pressure loss increases rapidly, in particular at the highertemperatures which are generally necessary for regeneration.

Example 2

The preceding example was repeated with a bed depth of 12 m. The reactorthen contained in total 6720 g of catalyst. The results are shown inFIG. 2 as pressure loss vs. weight-based residence time. It can be seenthat for weight-based residence times of the less than 2 hours, thepressure loss increases rapidly and, as expected, to an even greaterextent than in Example 1.

Example 3

40 g of a spent TS-1 catalyst (removed after an operating time of about600 hours) in the form of pellets with a diameter of 1.5 mm wereintroduced into an electrically heated stainless-steel tube having aninternal diameter of 25 mm and a length of 200 mm. After drying at 50°C. in a stream of nitrogen, this removed catalyst contained 1.0% byweight of carbon. For monitoring the internal temperature, athermocouple was mounted in the center of the catalyst bed. 61(s.t.p.)/h of nitrogen were passed through this bed. The heating wasthen switched on, and the temperature increased to 450° C. over thecourse of 84 minutes. When the temperature was reached, air was slowlymetered in (from 0 to a maximum of 11 (s.t.p.)/h over the course of 50minutes). The regeneration was subsequently carried out with 61(s.t.p.)/h of nitrogen and 1 (st.p.)/h of air 1 hour at 450° C. Theweight-based residence time of the regeneration gas, defined asindicated in the description, was 5.3 hours before and 4.6 hours afterthe stream of air was switched on. The heating was subsequently switchedoff and, in order to accelerate cooling, the stream of nitrogenincreased to 101 (s.t.p.)/h. The change in temperature and the amountsof nitrogen and air employed are shown in FIG. 3.

The maximum temperature peak observed was 10° C. (at 157 and 191 min).After cooling, the catalyst was removed and analyzed. The carbon contentwas <0.1% by weight. The regenerated catalyst exhibited the sameactivity and selectivity in the epoxidation of propene using hydrogenperoxide in methanol as did the fresh catalyst.

Example 4

800 g of a spent TS-1 catalyst (removed after an operating time of about1000 hours) in the form of pellets having a diameter of 1.5 mm wereintroduced into an electrically heated stainless-steel tube having aninternal diameter of 40 mm and a length of 2100 mm. After drying at 50°C. in a stream of nitrogen, this removed catalyst contains 1.2% byweight of carbon. In order to monitor the internal temperature, the tubewas fitted with thermocouples at separations of about 200 mm. A gaseousstream composed of 100 l (s.t.p.)/h of nitrogen and 30 l (s.t.p.)/h ofair (corresponds to 130 l (s.t.p.)/h with 4.6% by volume of oxygen innitrogen) was passed through this bed. The weight-based residence timeof the regeneration gas, defined as indicated in the description, was4.9 hours. The pressure loss over the bed was about 20 mbar. The heatingwas subsequently switched on and the temperature increased to 400° C.over the course of 2 hours. The pressure loss over the bed increasedabout 20 mbar. The temperature was then held at 400° C. for a further 8hours. The maximum temperature observed within the bed (hot spot) wasonly 425° C. After cooling, the catalyst was removed and analyzed. Thecarbon content was <0.1% by weight. The regenerated catalyst exhibitedthe same activity and selectivity in the epoxidation of propene usinghydrogen peroxide in methanol as did a fresh catalyst.

1. A process for the regeneration of a zeolite catalyst, comprising: treating the catalyst thermally in a fixed-bed reactor in the presence of a gas stream at temperatures above 120° C., wherein the weight-based residence time of the gas stream over the catalyst during the thermal treatment is from 4 to 6 hours, wherein the fixed-bed reactor is a tubular reactor or a tube-bundle reactor.
 2. A process as claimed in claim 1, where the zeolite catalyst is a titanium silicalite having the TS-1 structure.
 3. A process as claimed in claim 1, where the regeneration gas comprises from 0.1 to 10% by volume of oxygen.
 4. A process as claimed in claim 1, which comprises: (1) washing the zeolite catalyst with a solvent, (2) drying the washed zeolite catalyst at a temperature of from −50 to 250° C., (3) heating the dried catalyst, and (4) regenerating the heated catalyst as described in claim
 1. 5. A process as claimed in claim 4, which further comprises at least one of the following (5) and (6), which are carried out after (4): (5) cooling the regenerated catalyst obtained in (4), (6) conditioning the catalyst obtained in (4) or in (5) wherein, for conditioning, small amounts of a solvent are admixed to an inert conditioning gas stream passing the catalyst and the inert conditioning gas stream containing solvent is passed through the catalyst bed.
 6. An integrated process for the oxidation of an alkene, comprising: reacting the alkene with a hydroperoxide in the presence of a zeolite catalyst, and subsequently, regenerating the catalyst by means of a process as described in claim
 1. 7. A process as claimed in claim 6, wherein the alkene is propene and the hydroperoxide is hydrogen peroxide.
 8. A process as claimed in claim 6, where the regenerated catalyst is re-employed for the reaction of the alkene with the hydroperoxide.
 9. A process as claimed in claim 8, where the alkene is propene and the hydroperoxide is hydrogen peroxide.
 10. A process for the regeneration of a zeolite catalyst, comprising: treating the catalyst thermally in a fixed-bed reactor in the presence of a gas stream at temperatures above 120° C., wherein the weight-based residence time of the gas stream over the catalyst during the thermal treatment is from 3 to 10 hours.
 11. An integrated process for the oxidation of an alkene, comprising: reacting the alkene with a hydroperoxide in the presence of a zeolite catalyst and subsequently treating the catalyst thermally in a fixed-bed reactor in the presence of a gas stream at temperatures above 120° C., wherein the weight-based residence time of the gas stream over the catalyst during the thermal treatment is from 4 to 6 hours, to regenerate the catalyst, wherein the fixed-bed reactor is a tubular reactor or a tube-bundle reactor.
 12. The integrated process as claimed in claim 11, wherein the regenerated catalyst is re-employed for the reaction of the alkene with the hydroperoxide, and wherein the alkene is propene and the hydroperoxide is hydrogen peroxide.
 13. An integrated process as claimed in claim 11, wherein the pressure loss over the reactor in which the catalyst is regenerated is not greater than 4 bar. 